Physiology
Nerve and Muscle Physiology
Nerve and muscle physiology for MBBS and NEET-PG: action potential, nerve conduction, neuromuscular junction and the sliding filament theory of contraction, mapped to NMC codes PY3.
MedNext Academy | 3 min read
Nerve and Muscle Physiology
Nerve and muscle physiology for MBBS and NEET-PG: action potential, nerve conduction, neuromuscular junction and the sliding filament theory of contraction, mapped to NMC codes PY3.
Nerve and muscle physiology explains how signals are generated and how movement is produced. It covers the resting membrane potential and action potential, nerve conduction and fibre types, the neuromuscular junction, and the sliding filament mechanism of skeletal and smooth muscle contraction.
High-yield: Nerve and Muscle Physiology
- The resting membrane potential of a neuron is near minus 70 millivolts and depends on potassium permeability.
- An action potential is all or none and follows the threshold being reached.
- Depolarisation is driven by sodium entry and repolarisation by potassium exit.
- The absolute refractory period ensures the action potential travels one way and limits the maximum firing rate.
- Myelinated fibres conduct faster through saltatory conduction, where the impulse jumps between nodes of Ranvier.
- Nerve fibres are classified as A, B and C, with A alpha being the largest and fastest.
- At the neuromuscular junction acetylcholine binds nicotinic receptors and is broken down by acetylcholinesterase.
- The sliding filament theory explains contraction as actin sliding over myosin without the filaments themselves shortening.
- Excitation-contraction coupling depends on calcium release from the sarcoplasmic reticulum after membrane depolarisation.
- In skeletal muscle calcium binds troponin C, which moves tropomyosin and exposes the myosin binding sites on actin.
- A motor unit is one motor neuron and all the muscle fibres it supplies.
- Summation and recruitment increase the force of contraction, and tetanus is sustained maximal contraction.
- Smooth muscle contraction is triggered by calcium binding calmodulin and activating myosin light chain kinase.
- The length-tension relationship shows that muscle generates maximum force at its optimal resting length.
Action potential phases
- **Resting potential:** About minus 70 millivolts, set by potassium permeability.
- **Depolarisation:** Sodium influx through voltage-gated channels.
- **Repolarisation:** Potassium efflux returns the potential toward rest.
- **Refractory period:** Absolute period ensures one-way conduction; relative allows a strong stimulus.
NMC competencies in this chapter
- **PY3.1:** Resting membrane potential and action potential
- **PY3.3:** Nerve fibre types and their properties
- **PY3.5:** Conduction of the nerve impulse
- **PY3.7:** The neuromuscular junction and its transmission
- **PY3.10:** Structure of skeletal muscle and the sliding filament theory
- **PY3.12:** Excitation-contraction coupling
- **PY3.15:** Types of muscle contraction and their mechanics
- **PY3.17:** Properties of smooth muscle
Frequently Asked Questions
Why is the action potential described as all or none?
Once the stimulus reaches threshold the action potential fires with a fixed size and shape, and a stronger stimulus does not make it larger. Below threshold no action potential occurs.
How does myelin speed up conduction?
Myelin insulates the axon so the impulse jumps between the exposed nodes of Ranvier, a process called saltatory conduction that is much faster than continuous conduction.
What is the role of calcium in muscle contraction?
Calcium released from the sarcoplasmic reticulum binds troponin C in skeletal muscle, which moves tropomyosin off the actin binding sites so that myosin can pull the actin filaments and shorten the muscle.
What is a motor unit?
A motor unit is a single motor neuron together with all the muscle fibres it innervates, and it is the smallest functional unit of muscle contraction.
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